Superalloy having improved sulfidation resistance

ABSTRACT

An alloy being resistant to sulfidation attack having a nickel base and chromium in an amount not more than about 13 percent, the sulfidation resistance being supplied by the addition of up to 1 percent of vanadium, or tantalum, or both.

United States Patent Inventors Richard J. Quigg;

Scott T. Scheirer, both of Euclid, Ohio 570,395

Aug. 5, 1966 Oct. 26, 1971 TRW Inc.

Cleveland, Ohio Appl. No. Filed Patented Assignee SUPERALLOY HAVING IMPROVED [56] References Citedl UNITED STATES PATENTS 3,093,476 6/1963 Gittus 75/171 3,164,465 1/1965 Thielemann 75/171 3,167,426 1/1965 Freche et a1. 75/171 3,293,030 12/1965 Child et a1. 75/171 3,322,534 5/1967 Shaw et a1 75/171 Primary Examiner-Richard 0. Dean Atlorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: An alloy being resistant to sulfidation attack having a nickel base and chromium in an amount not more than about 13 percent, the sulfidation resistance being supplied by the addition of up to 1 percent of vanadium, or tantalum, or both.

SUPERALLOY HAVING IMPROVED SULMDATIUN RESISTANCE The present invention relates to nickel base superalloys having excellent stress rupture properties at elevated tempera tures, resisting thermal fatigue, and being resistant to oxidative corrosion.

The development of jet aircraft engines operating at higher and higher temperatures has made it mandatory to provide superalloys which have the ability to operate under extreme conditions of temperature and stress. In response to this requirement, various nickel-chromium base alloys have been suggested for high-temperature applications, but many of these are deficient in various physical respects. in some cases, they are deficient in stress rupture properties at elevated temperatures. in other cases they are unacceptable because of their extremely limited ductility, that is, they are quite brittle and difficult to machine. Still other alloys which would otherwise be satisfactory for use in high-temperature work have limited applicability because of the difficulties encountered in casting or forging such alloys.

In addition to the many stringent requirements posed by high-temperature service in jet engines, a new source of corrosion has been recently identified and analyzed. This corrosion is referred to as corrosion by sulfidation attack and occurs by the joint action of sulfur and alkaline and alkali metal salts, particularly sea salt. In the environment of a jet engine, the sulfur can be introduced by means of the jet fuel which frequently contains appreciable amounts of this element. The alkaline and alkali metal salts can be ingested into the engine from the environment, particularly in the case of aircraft which operate near bodies of sea water.

While the mechanism of the sulfidation corrosion has not been fully developed, it appears that such corrosion may proceed along the following lines. At temperatures on the order of 1,400" and l,900 F., the presence of the sulfur and the alkali metal salt leads to the formation of a molten slag containing sodium sulfate. This molten material serves to destroy the protective oxide film which exists on the superalloy at these temperatures. A reaction then ensues which results in the fonnation of a nickel sulfide-nickel eutectic. This is followed by the depletion of chromium from the alloy by the formation of a chromium sulfide. Finally, oxidation of the nickel sulfide-nickel eutectic and the chromium depleted alloy can occur, sometimes with catastrophic results.

Nickel base superalloys containing 15 or more chromium are not as susceptible to sulfidation attack as are nickel alloys containing less than 15% chromium. However, alloys containing l% or more chromium have been found to have physical properties at high temperatures which are not as desirable as the physical properties of nickel base alloys containing l3% chromium or less. Below the 13% chromium level, however, the sulfidation problem can become severe.

One of the objects of this invention is to provide an improved alloy having enhanced sulfidation attack resistance.

A further object of the invention is to provide a nickel base superalloy which has excellent physical properties at high temperatures, and which also has enhanced sulfidation attack resistance.

A further object of the invention is to provide an improved nickel base superalloy which has good casting and forgeability properties along with the other desirable properties mentioned.

Another object of the invention superalloy which has a balance of properties, including sulfidation resistance, not previously achieved in commercially available alloys heretofore used in similar environments.

The improvements of the present invention are particularly applicable to the type of superalloy which has been described in the application of Richard J. Quigg, Ser. No. 289,915, filed June 24, 1963, and issued as US. Pat. No. 3,254,994, on June 7, 1966. The alloy as described in that application had a properly correlated chemistry which provides excellent stress rupture properties, specifically a stress rupture life of at least 100 hours at L300 F. at an applied stress of 24,000 pounds per square inch.

is to provide a nickel base The overall range of compositions described in the aforementioned application is recited in the following table:

Within this broader range, the best alloys which have been produced have analyses within the following narrower ranges:

Chromium 9.541.095

Cobalt 9.0-] l .0%

Tungsten Eli-9.5%

Aluminum 60-68% Titanium 0.754.251:

Columbium l.2-l.0%

Carbon 0.085-0. l4%

Boron 0.020.04%

Zirconium 0.070. l 3% Nickel Substantially the bulnnce Of all of the alloys produced according to the aforementioned application, the following specific alloy appears tohave the best overall properties:

As discussed in the foregoing application, it has been found that the sum of the aluminum and titanium contents must be controlled to the range of 6.5 to 9% or preferably in the range of 7.25 to 8.25% if the proper microstructure and properties are to be achieved. it is believed that the improved properties of these alloys are attributable, at least in part, to the production of an intermetallic compound having the following formula:

Ni; A1 Ti where it plus y equals l, but y is not more than This interrnetallic compound is a face-centered cubic structure which constitutes the gamma-prime phase of the nickelaluminum phase diagram. When too much titanium plus aiu minum is present, and insufficient nickel is present to provide the intermetallic compound designated above, the alloy in its as-cast condition evidences spherulites of the primary gammaprime phase which endows the resulting alloy with a brittle structure. Even when the correct range of aluminum plus titanium content is observed, it is important that the titanium concentration not exceed 60% of the total aluminum plus titanium on a molar basis, as otherwise needlelike particles are produced in the microstructure which detract from the physical properties.

it is also important that the nickel-aluminum-titanium phase exist in a properly strengthened solid solution matrix. This strengthening is provided primarily by the relatively high tungsten content of the new alloys although some contribution is made in this regard by the presence of the columbium and to some extent by the cobalt present.

The alloys described above have excellent physical proper ties at elevated temperatures, and are capable of achieving a minimum stress rupture life of hours at l,800 F. at a stress of 24,000 pounds per square inch. When tested for sulfidation resistance, however, alloys of this type appear to be quite prone to weight loss under the test conditions. in this connection, we have developed a screening test for determining resistance to sulfidation. In this test, a cylindrical slug of the alloy to be tested, measuring inch in diameter and being 1 78 inch long is immersed halfw y in a silica ing a mixture of 99% sodium sulfate and l% sodium chloride maintained at a temperature of l,800 F. Thesample is kept in the fused salt mixture for a period of 1 hour after which it is cooled, and then descaled with cathodic protection in molten sodium hydroxide. The sample is then weighed and the weight loss per unit area is determined. We do not mean to imply that the test procedure described above is an exact simulation of the conditions existing in a jet engine operating under sulfidation conditions; however, we have found this test to be a reliable screening test wherein the results closely parallel the results obtained when testing is performed in an elaborate corrosion rig.

We have now found that the addition of controlled amounts -of vanadium and tantalum to nickel base superalloys having 13% or less chromium very substantially improves the sulfidation resistance of the alloy. We have found that additional benefits can be derived particularly with respect to the workability and oxidation resistance of the alloy when used in wrought form by the addition of controlled amounts of hafnium. Specifically, we have found amounts up to 1% of each of the three elements to be effective for the purposes intended with the addition of 0.5% of each forming the particularly preferred species of the invention.

Through the addition of the controlled amounts of vanadium and/or tantalum, we have been able to reduce the weight loss due to sulfidation in the test described above, to values far below those which were characteristic of the alloys before the addition of one or both of these elements. We have been able, for example, to consistently reduce the weight loss due to sulfidation to a value of about 3.0 grams per square decimeter or less, and frequently to substantially below 1 gram per square decimeter. in contrast, the alloys not containing the vanadium and/or tantalum addition but otherwise having the same nominal analyses frequentlyevidenced weight losses substantially in excess of grams per square decimeter under the same test conditions. A comparison of sulfidation resistance values for alloys including and not including the improvements of the present invention as given in the following table:

Weight per loss,

Alloy Analysis g. l sq. drn.

' The alloys of the present invention can be used in t he is cast as well as in the wrought condition, since they are castable under vacuum conditions and sufficiently ductile to be workable. In the wrought condition, it is desirable to provide grain sizes ranging from about 3 to 5 on the ASTM scale. The crystallographic orientation of the alloys does not appear to have a significant effect on their sulfidation resistance.

From the foregoing, it will be understood that the sulfidationresistant allo of the present invention have properties particularly usefu for parts for jet engines operating in an environment of sulfidation. It should also be evident that various modifications can be made to the described embodiment without departing from the scope of the present invention.

We claim as our invention:

1. A sulfidation attack resistant alloy composition consisting essentially of from 9 to 13% chromium, from 5 to l5% cobalt, from 7 to 12% tungsten, from 5,5 to 7% aluminum, from 0.5 to 2.0% titanium, up to 2% columbium, from 0.05 to 0.25% carbon, from 0.01 to 0.08% boron, from 0.01 to 0.20% zirconiurn, about 0.5% vanadium and about 0.5% tantalum, the balance being essentially nickel, said alloy evidencing a weight loss of no more than about 3 grams per square decimeter when maintained in a molten mixture of 99% sodium sulfate and 1% sodium chloride at l,800 F. for 1 hour.

2. A sulfidation attach resistant alloy composition consisting essentially of from 9.5 to l 1.0% chromium, from 9.0 to l 1.0% cobalt, from 8.5 to 9.5% tungsten, from 6.0 to 6.8% aluminum, from 0.75 to 1.25% of titanium, from 1.2 to 1.8% columbium, from 0.085 to 0.14% carbon, from 0.02 to 0.045 boron, from 0.07 to 0.13% zirconium, about 0.5% vanadium and about 0.5% tantalum and the balance being essentially nickel, said alloy evidencing a weight loss of no more than about 3 grams per square decimeter when maintained in a molten mixture 99% sodium sulfate and 1% sodium chloride at l .800 F. for 1 hour. 

2. A sulfidation attach resistant alloy composition consisting essentially of from 9.5 to 11.0% chromium, from 9.0 to 11.0% cobalt, from 8.5 to 9.5% tungsten, from 6.0 to 6.8% aluminum, from 0.75 to 1.25% of titanium, from 1.2 to 1.8% columbium, from 0.085 to 0.14% carbon, from 0.02 to 0.045 boron, from 0.07 to 0.13% zirconium, about 0.5% vanadium and about 0.5% tantalum and the balance being essentially nickel, said alloy evidencing a weight loss of no more than about 3 grams per square decimeter when maintained in a molten mixture 99% sodium sulfate and 1% sodium chloride at 1,800* F. for 1 hour. 